Vanadyl hydrogen phosphate hemihydrate, VOHPO4.0.5H2O, has tremendous technological importance as the precursor to the vanadyl pyrophosphate, (VO)2P2O7 which is used as the catalysts for the commercially established selective oxidation of butane to maleic anhydride. Maleic anhydride is a commercially important chemical intermediate for acquiring unsaturated polyester, succinic anhydride, gamma-butyrolactone, 1,4-butanediol, tetrahydrofuran, fumaric acid, malic acid and d,l-tartaric acid. Vanadyl hydrogen phosphate hemihydrate is known to be crucially in determining the catalytic activity and selectivity of the produced vanadyl pyrophosphate. In fact, it is the surface area of the produced vanadyl pyrophosphate that affects the catalytic activity and selectivity because higher surface area implies higher number of active sites per unit mass of catalysts. Thus, the higher surface area catalysts can operate at lower temperatures which lead to enhance selectivity.
U.S. Pat. No. 4,333,853 is an application regarding to a method to prepare vanadium phosphorus oxide catalysts by reducing pentavalent vanadium compounds and pentavalent phosphorus compounds together under reflux. In this disclosed US application, the reflux needed to be conducted for at least 16 hours to finally acquire the vanadyl hydrogen phosphate hemihydrate and no dopant is suggested to be used to enhance the catalytic activity of the produced vanadium phosphorus oxide catalysts. Milberger et al. filed another patent application in U.S. Pat. No. 4,360,453. In this application, Milberger et al. suggested an extraction method to recover the produced catalyst precursors, vanadyl hydrogen phosphate hemihydrate, by dissolving the produced precursors into water. The dissolved precursors were then recovered via evaporation or spray drying. Moreover, this disclosed method employed gases such as HBr and HCl as the reducing agent by passing these gases through the organic media, thus no reflux of the organic media is needed in this method.
U.S. Pat. Nos. 4,132,670 and 4,187,235 disclosed another method to prepare vanadium phosphorus oxide catalysts. The disclosed method includes two different stages of reflux which at first reduces the pentavalent vanadium-containing compounds, then reflux the reduced vanadium-containing compounds with orthophosphoric compounds thus forming the catalyst precursors, vanadyl hydrogen phosphate hemihydrate. These applications have suggested in using mixture of vanadium with others rare metal in preparing the vanadium phosphorus oxide by refluxing the mixture at the first stage of reflux. In order to produce high surface area catalysts, inventors of the present application found that utilization of other metal or dopants shall be involved in second stage of reflux for a short period of time instead of as proposed by the aforementioned US applications.
On the other hand, U.S. Pat. No. 4,562,268 is another improved method to produce vanadium phosphorus oxide catalysts. This application has claimed employing an alcohol modifying agent in the reducing process which leads to production of catalysts with high surface area.
U.S. Pat. No. 4,569,925 also filed an application on reacting vanadium-containing compound with phosphorus-containing compounds in a liquid organic media to produce the precursors, and then activate the precursors to form the vanadium phosphorus oxide catalysts with a type of inert air and at least one hydrocarbon. In this US application, all the reactants are reflux in a single reaction to produce the precursor.